Through retrieving existing technologies, the following literatures are retrieved:
The fuel cell power supply control principle publicized by the invention patent application of China called “power distribution method for fuel cell mixed power system” with application number “200310103253.3”: Adopt SOC calculation to control according to measurement load control signal (such as throttle signal) and power cell SOC (state of charge) the output of the fuel cell DCDC to satisfy the energy demands of the load system, fuel cell system and power cell pack under the state of charge.
The fuel cell power supply control method publicized by the invention patent application of China called “fuel cell based mixed power device energy management system” with application number “201010108281.4” also adopts SOC calculation, where,
Here is the calculation formula for the state of charge (SOC):soc(k)=(BC×soc(k−1)−∫k-1kioutdt+∫k-1kioutdt)/BC 
In the above calculation formula, BC represents cell capacity, soc(k) represents the SOC value of cell at current moment, soc(k−1) represents the SOC value at the previous moment, iout represents cell discharging current and iin represents cell charging current.
It is known from the above formula that the SOC calculation is a kind of algorithm to obtain the state of charge (SOC) of battery according to the battery current data collected, the cell capacity data set, based on the integration algorithm and by correcting according to the actual cell capacity, cell voltage, temperature at the time of actual use. That invention application has the following disadvantages:
1. All the above control method relies on SOC calculation; and the SOC calculation relies on accurate current data, the accuracy of current data depends on the accuracy, sensitivity, stability of current measurement device; however, the current measurement device also has an error; therefore, the SOC calculation method can only be an approximate estimation of the state of charge of the energy storage device. The existing fuel cell system on board vehicle using the SOC calculating method adopts a dual-range current sensor in order to obtain a relatively accurate current value; however, a dual-range current sensor is unable to cover the whole range and at the same time is also unable to avoid the zero drift that the current sensor has, therefore, the current sensor has to be calibrated frequently. In this circumstance, a fuel cell company, after selling a fuel cell system on board vehicle, has to calibrate regularly the current measurement device sensor. The product immaturity will directly influence the mercerization progress of fuel cell vehicles.
2. The capacity of the energy storage device (battery) may reduce with use gradually. It is known from the formula that in order to obtain SOC accurately, it is imperative to have an accurate capacity value of the energy storage device. Therefore, it is imperative to calibrate the capacity of the energy storage device (battery), which can only be a vague estimation. Therefore, it is unable to accurately conduct the fuel cell system energy management by adopting the SOC calculation method.
3. The current output fluctuation amplitude is large when a forklift is working.
The voltage of the energy storage device (battery) used on fuel cell bus, fuel cell car as auxiliary power is often hundreds of volts, the current range is from negative tens of amperes to positive tens of amperes; under the circumstance that the current range is small, the accuracy of battery current value is relatively high, under this working condition, though the use of the SOC calculation method is not so good as the said fuel cell mixed power supply energy management method, it is barely satisfactory.
The voltage of the energy storage device (battery) used on fuel cell forklift as auxiliary power is often tens of volts, but the current range fluctuates largely. For example, the common nominal voltage 24V corresponds to a working current range −500˜500 A; the nominal voltage 36V corresponds to a working current range −800˜1000 A, the nominal voltage 48V corresponds to a current range −600˜800 A. This is because when a fuel cell forklift is working, it constantly lifts loads, drives at an accelerated speed, brakes, etc. that result in the output current of the battery increasing from several amperes gradually to hundreds of amperes and even a thousand amperes and turning from outputting a thousand amperes to inputting hundreds of amperes. As the current range is large, it is very difficult to measure the current value accurately; at the same time, that the current output fluctuation frequency is high when a forklift is working further makes real-time and accurate current measurement become very difficult; and SOC integration algorithm can also amplify the deviation constantly. Therefore, it is unable to realize an accurate fuel cell system energy management by adopting the SOC calculation method on a fuel cell forklift.
4. Energy recovery issue, protection issue.
When a fuel cell vehicle with an energy recovery system (such as the invention patent application called “power distribution method for fuel cell mixed power system” with application number “200310103253.3”) brakes for energy recovery, the energy resulting from braking is input in the energy storage device with a current being often as high as hundred of amperes and even up to 1000 A in some cases, then the voltage of the energy storage device will increase sharply, at the same time, the internal resistances of cables, connections, relays, etc. in the circuits through which current passes at recovery braking can all cause the vehicle voltage to rise; if the battery voltage exceeds the protection voltage of the energy storage device, or the vehicle voltage exceeds the protection voltage of the vehicle, the system or vehicle may disconnect the relay making external connection to realize equipment protection. As a result of disconnecting the relay, the energy storage device is unable to continue to absorb the braking energy and braking can not proceed normally. The vehicle may be out of control and even have an accident. In order that at energy recovery, the voltage of the energy storage device does not exceed the protection voltage of the energy storage device, or the vehicle voltage does not exceed the protection voltage of the vehicle, it is imperative to control the actual state of charge (SOC) of the energy storage device to be a right or a lower value.
However, as the SOC calculation is based on the measured battery current value and the actual battery capacity and as the battery current data, the actual battery capacity can not be measured accurately, it results in the SOC calculation method being unable to obtain the actual SOC values. When the SOC measurement value is lower than the actual value, the actual state of charge (SOC) of the energy storage device is at a high value, the voltage of the energy storage device will exceed the protection voltage of the energy storage device or the protection voltage of the vehicle; this will constitute a safety hazard to the fuel cell vehicle.
The said fuel cell mixed power supply energy management method is to control the output current of the DCDC converting unit, respond to the energy demand resulting from load condition change and at the same time ensure the energy storage device to be in a best state of charge according to the measured voltage of the energy storage device and the actual current output by the DCDC converting unit under the circumstance without connecting the vehicle operation input signal (throttle, brake) and calculating SOC.